The present invention relates generally to error detector circuitry for detecting errors of discretely-encoded signals transmitted to a receiver and, more particularly, to a bad frame indicator for detecting bad frames of information signals received by a receiver constructed to receive discretely-encoded signals comprised of coded frames.
A communication system is comprised, at a minimum, of a transmitter and a receiver interconnected by a transmission channel. A communication signal is transmitted by the transmitter upon the transmission channel, thereafter to be received by the receiver. A radio communication system is a communication system in which the transmission channel comprises a radio frequency channel defined by a range of frequencies of the electromagnetic frequency spectrum. A transmitter operative in a radio communication system must convert the communication signal into a form suitable for transmission thereof upon the radio frequency channel.
Conversion of the communication signal into a form suitable for transmission thereof upon the radio frequency channel is effectuated by a process referred to as modulation. In such a process, the communication signal is impressed upon an electromagnetic wave. The electromagnetic wave is commonly referred to as a "carrier signal." The resultant signal, once modulated by the communication signal, is commonly referred to as a modulated carrier signal or, more simply, a modulated signal. The transmitter includes circuitry operative to perform such a modulation process.
Because the modulated carrier signal may be transmitted through free space over large distances, radio communication systems are widely utilized to effectuate communication between a transmitter and a remotely-positioned receiver.
The receiver of the radio communication system which receives the modulated carrier signal contains circuitry analogous to, but operative in a manner reverse with that of, the circuitry of the transmitter and is operative to perform a process referred to as demodulation.
Numerous modulated carrier signals may be simultaneously transmitted as long as the signals are transmitted upon differing radio frequency channels defined upon the electromagnetic frequency spectrum. Regulatory bodies have divided portions of the electromagnetic frequency spectrum into frequency bands and have regulated transmission of the modulated carrier signals upon various ones of the frequency bands. (Frequency bands are further divided into channels, and such channels form the radio frequency channels of a radio communication system.)
A two-way radio communication system is a radio communication system, similar to the radio communication system above-described, but which permits both transmission of a modulated carrier signal from a location and reception at such location of a modulated carrier signal. Each location of such a two-way communication system contains both a transmitter and a receiver. The transmitter and the receiver positioned at a single location typically comprise a unit referred to as a radio transceiver or, more simply, a transceiver.
A cellular, communication system is one type of two-way radio communication system in which communication is permitted with a radio transceiver positioned at any location within a geographic area encompassed by the cellular, communication system.
A cellular, communication system is created by positioning a plurality of fixed-site radio transceivers, referred to as base stations, at spaced-apart locations throughout a geographic area. The base stations are connected to a conventional, wireline, telephonic network. Associated with each base station of the plurality of base stations is a portion of the geographic area encompassed by the cellular, communication system. Such portions are referred to as cells. Each of the plurality of cells is defined by one of the base stations of the plurality of base stations, and the plurality of cells together define the coverage area of the cellular, communication system.
A radio transceiver, referred to in a cellular communication system as a cellular radiotelephone or, more simply, a cellular phone, positioned at any location within the coverage area of the cellular communication system, is able to communicate with a user of the conventional, wireline, telephonic network by way of a base station. Modulated carrier signals generated by the radiotelephone are transmitted to a base station, and modulated carrier signals generated by the base station are transmitted to the radiotelephone, thereby to effectuate two-way communication therebetween. (A signal received by a base station is then transmitted to a desired location of a conventional, wireline network by conventional telephony techniques. And, signals generated at a location of the wireline network are transmitted to a base station by conventional telephony techniques, thereafter to be transmitted to the radiotelephone by the base station.)
Increased usage of cellular, communication systems has resulted, in some instances, in the full utilization of every available transmission channel of the frequency band allocated for cellular radiotelephone communication. As a result, various ideas have been proposed to utilize more efficiently the frequency band allocated for radiotelephone communication. By more efficiently utilizing the radio frequency band utilized for radiotelephone communication, the transmission capacity of an existing, cellular, communication system may be increased.
Certain of such ideas involve converting a communication signal into discrete form prior to modulation and transmission thereof by a transmitter upon the communication channel. By converting the communication signal into such discrete form, a modulated carrier signal formed therefrom may be transmitted in short bursts, and more than one modulated signal may be transmitted sequentially upon a single transmission channel.
Converting the communication signal into discrete form is typically effectuated by an encoding technique, and apparatus which effectuates such conversion is typically referred to as a source encoder. An encoded signal generated as a result of an encoding technique is typically in the form of a discrete binary data stream. The elements (i.e., bits) of the discrete binary data stream represent various characteristics of the information signal.
A transmitter which transmits such a discretely-encoded signal typically further includes a channel encoder coupled to receive the encoded signal generated by the source encoder. The channel encoder is operative to increase the redundancy of the signal, and such redundancy facilitates accurate determination of the signal once received by a receiver.
As a radio frequency channel is not a noise-free transmission channel, noise and other transmission difficulties (e.g., intersymbol interference and Rayleigh fading) may cause a receiver to receive a signal other than that which was transmitted by the transmitter. Because an encoded signal, encoded by a channel encoder, contains redundancies, the receiver is better able to determine accurately the actual information signal transmitted by a transmitter, even when the modulated signal formed of the encoded signal has been distorted during transmission thereof. Channel decoder circuitry of the receiver is operative to remove the redundancies introduced upon the signal by the channel encoder of the transmitter.
Various block and convolutional coding and decoding techniques have been developed to facilitate accurate recreation of a communication signal. One such convolutional coding technique is a Viterbi coding technique. A transmitter may, e.g., include a Viterbi coder to perform channel encoding and a receiver may, e.g., correspondingly, include a Viterbi decoder to perform channel decoding.
When, however, distortion introduced upon the transmitted signal is so significant that the receiver receives bursts of significantly-distorted information, decoder circuitry of the receiver is unable to decode properly the received signal. Such incorrect decoding of the received signal results in the receiver recreating a signal other than the communication signal forming the modulated signal transmitted by the transmitter.
Parity bits are sometimes utilized in communication systems which transmit modulated signals formed of discretely-encoded communication signals. The values of parity bits received by a receiver are utilized to provide an indication of the amount of distortion introduced upon a signal during transmission thereof.
A transmitter operative to transmit the discretely-encoded signal oftentimes includes circuitry for introducing the parity bits into the encoded signal transmitted by the transmitter. The receiver which receives the transmitted signal including such parity bits makes a determination as to whether the parity bits of the signal received by the receiver correspond to a predetermined sequence of values (wherein such predetermined sequence of values correspond to a sequence of parity bits actually transmitted by the transmitter).
A significant amount of distortion introduced upon a signal is indicated when the receiver receives the encoded signals having the parity bits which are of values different than the predetermined sequence of values. Portions of the received signal located about the distorted parity bits are ignored by the receiver as distortion of the parity bits is indicative of distortion introduced upon other bits of that portion of the signal.
However, by random process, the parity bits received by the receiver may be of values indicative of an undistorted signal even when the signal is significantly distorted. In such instances, a receiver incorrectly determines that a distorted signal has not been distorted during transmission thereof and thereby recreates an erroneous signal. Such a method, based upon detection of distorted parity bits, of determining whether a signal received by a receiver is significantly distorted is, therefore, inadequate.
If receiver circuitry of the receiver incorrectly makes a determination as to the amount of distortion introduced upon a signal transmitted thereto, a recreated signal generated by the receiver is typically more than distorted and is, rather, more frequently, audibly noticeable as squelching.
What is needed, therefore, is a more accurate system of determining when significantly-distorted signals received by a receiver should be rejected by the receiver.